Effects of auditory processing training on speech perception and brainstem plastisity in adolescents with autism spectrum disorders

Objective Autism spectrum disorder (ASD) is a neurodevelopmental disorder. A major problem of ASD is speech perception impairment in the presence of background noise. Additionally, researchers have reported temporal auditory processing impairment in patients with ASD. In the present study, we evaluated the effects of a temporal-based training program on improvement of speech perception in the presence of noise using the speech auditory brainstem response (sABR). Materials & Methods Twenty-eight adolescents with high functional ASD with the mean age of 14.35±1.86 years were randomly selected and divided into an ASD group (11 males and three females) and a control group (13 males and one female). All the subjects had a normal hearing and intelligence threshold and had no history of neurological disorder. A speech perception test was performed in signal-to-noise ratios of 0 and +10. The intervention group received a temporal processingbased auditory training program, and the control group received a conventional training program. A P-value of <0.05 was considered significant. Results After training, speech perception in the presence of noise was significantly higher (P <0.001) and the latency of all sABR waves was lower in the intervention group compared to the control group. Conclusion Improvement of speech perception in noisy environments and the reduced latency of sABR waves following a temporal processingbased training program highlight the role of brainstem neural plasticity in speech processing.


Introduction
Autism spectrum disorder (ASD) is a neurodevelopmental disorder with a higher frequency in boys than girls (4:1). The prevalence of this disorder in 2014 was reported one in 88 children in the United States (1). One of the main problems of children with ASD is impaired speech perception in the presence of background noise (2), which significantly affects communication skills (3). In everyday life, we are exposed to a variety of environmental noises and the ability to communicate in the presence of noise is an essential skill for successful participation in educational and social environments (2). A spoken signal includes temporal signals such as rapid changes in duration, silent intervals, and rapid rises and drops in amplitude (4). Recognition of these temporal features of spoken signals leads to comprehension of linguistic symptoms, recognition of vowels from consonants, identification of specific consonants, and deduction of subtle signs (5). Therefore, the ability to decode speech to extract meaningful linguistic information is dependent on normal auditory processing.
Many researchers have reported normal or abnormal temporal processing in children with ASD. In addition, disturbances in the encryption and understanding of the temporal aspects of auditory stimuli (such as duration of stimuli and intervals between successive stimuli) have been reported in this group of children. Since temporal processing plays a significant role in speech perception in noisy environments, the problem of speech comprehension in noisy conditions in children with ASD can be due to a defect in temporal processing (6).
Numerous studies have shown neural plasticity in auditory pathways in animals and humans (7,8).
Evidence has shown that training programs could contribute to neurophysiological changes and improved hearing skills. Tallal et al. (1981) and Merzenich et al. (1996) who developed a bottom-up auditory-training program showed in their studies that by increasing the difficulty level of exercises, the individual's temporal auditory skills gradually improved (9,10). Therefore, if the impaired rapid processing of an auditory stimulus causes linguistic and verbal problems, bottom-up interventions, which emphasize the speed of stimulus processing, help to improve these problems. In the study of  (13,14).
Regarding the role of temporal auditory processing in speech perception in noisy environments, this study aimed to evaluate the effects of a temporal processing-based auditory training program on improving speech perception in noisy conditions in adolescents with high functional ASD. Moreover, the effectiveness of the applied training program was evaluated after one month.

Participants
The study was conducted on 28 adolescents with high functional ASD (24 males and five females) with an age range of 10 to 16 years (±1.86). The participants were selected randomly from special schools and evaluated by clinicians.
The inclusion criteria were having a normal hearing threshold (equal to or better than 25 dB in both ears at 250-8000 Hz (ANSI2004)) and having no history of neurological disorders. All the subjects provided informed consent. In case of unwillingness of the participants or their parents, they were excluded from the study.

Procedure
The present study was performed in signalto-noise ratios of 0 and +10, and scores were obtained in percentages (23,24 (14).

Statistical analysis
The normality of the data was first assessed using the Shapiro-Wilk test. A paired t-test was used for data analysis before and after the intervention to assess the data normality, and the covariance analysis test was used for the post-interventional results in the two groups.
A P-value less than 0.05 was considered statistically significant. All the statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) for Windows, version 10.

Results
In this part, the descriptive features (mean and to the control group in the post-test. In the ASD group, speech perception in noise was significantly higher after the training than before the training. Moreover, there was a significant difference in the speech perception score before and after the training in the both signal-to-noise ratios of 0 and +10; however, there were no significant differences tin the control group ( Table 1).
The latency of all the speech ABR waves was lower in the intervention group than in the control group. The mean latency of all waves in the two groups was similar before the implementation of the temporal-based training program, and there was no significant difference between the two groups ( Table 2). Further, no significant difference Iran J Child Neurol. Winter 2021 Vol. 15 No. 1 was observed between the two groups in terms of wave amplitude. In addition, the results showed that the performance of the intervention group did not differ one month after the intervention (Table   3).

Discussion
Individuals with ASD, despite their normal hearing ability, experience abnormal speech perception that is more pronounced in challenging environments and in the presence of competitive noise (27).
In this study, speech perception increased in the intervention group and improvement in the scores was even observed one month after the intervention. However, according to our knowledge, no study was carried out to improve speech perception in patients with ASD, and this is the first study attempting to improve speech perception by considering auditory processing, due to the major role of auditory processing in speech perception.
We investigated the effects of a bottom-up training program on brainstem neural plasticity and speech perception improvement in noisy environments.
In this case, although the results were clear, we suggest to conduct more detailed studies to further elucidate the impact of this training program.

In Conclusion
Improvement in speech perception in noisy environments and latency of speech ABR waves immediately after performing the training program and one month later suggests the role of temporal auditory processing in brainstem bottom-up pathways in speech perception.